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Re: [XEN v5 01/10] xen/arm: domain_build: Track unallocated pages using the frame number
- To: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Michal Orzel <michal.orzel@xxxxxxx>
- Date: Wed, 19 Apr 2023 14:05:44 +0200
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- Delivery-date: Wed, 19 Apr 2023 12:06:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hi Ayan,
On 13/04/2023 19:37, Ayan Kumar Halder wrote:
>
>
> rangeset_{xxx}_range() functions are invoked with 'start' and 'size' as
> arguments which are either 'uint64_t' or 'paddr_t'. However, the function
> accepts 'unsigned long' for 'start' and 'size'. 'unsigned long' is 32 bits for
> Arm32. Thus, there is an implicit downcasting from 'uint64_t'/'paddr_t' to
> 'unsigned long' when invoking rangeset_{xxx}_range().
>
> So, it may seem there is a possibility of lose of data due to truncation.
>
> In reality, 'start' and 'size' are always page aligned. And Arm32 currently
> supports 40 bits as the width of physical address.
> So if the addresses are page aligned, the last 12 bits contain zeroes.
> Thus, we could instead pass page frame number which will contain 28 bits
> (40-12
> on Arm32) and this can be represented using 'unsigned long'.
>
> On Arm64, this change will not induce any adverse side effect as the width of
> physical address is 48 bits.
NIT: This reads as const, so it would be better to write: "as the max supported
width of ..."
Thus, the width of 'gfn' (ie 48 - 12 = 36) can be
> represented using 'unsigned long' (which is 64 bits wide).
>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
With or without (after all this is just a commit msg):
Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
~Michal
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